Applicant makes self-expanding stents from nickel titanium shape memory alloy. The material is not particularly radiopaque and so stents made of it are usually provided with one or more radiopaque markers. Tantalum is an attractive material not only because it is biocompatible but also because it is close to the nickel titanium alloy in electrochemical potential and so resists galvanic corrosion after placement in the body. Furthermore, the two metals can be reliably welded together.
Applicant's WO 02/15820 discloses a particularly attractive form of radiopaque marker. When the self-expanding stent is radially compressed within a sheath in a delivery catheter system, the “ring of spoons” at each end of the stent, that serve as the tantalum markers, form a virtually complete ring of tantalum and so are relatively strongly visible to the radiographer.
An elegant aspect of the welding method taught in WO 02/15820 is that no jig or clamp is needed, to hold the two component parts in the desired relative positions for welding. Instead, the two components engage with each other in a mechanically interfering “form fit” ready for welding. Given the small size of the component, this is a distinct advantage.
Applicant's further published application, WO 01/58384 A1, discloses an elongate stent, preferably an oesophageal stent. A plurality of radiopaque beads are mounted onto selected ones of the uncovered extremities at the end or ends of the stent. In one embodiment, each bead has a throughbore which receives a spigot formed at the stent extremity. The spigots are each defined by two parallel resilient fingers formed out of an extension of the stent material. The two fingers are separated by a slit which allows the fingers to approximate for insertion along the throughbore, then to resiliently separate. Barbed tips are provided at the ends of the fingers to resist reverse movement of the fingers through the throughbore after they have emerged from the bead throughbore and again separated, thereby holding each bead on its spigot. To secure the beads, they are welded onto the spigots.
Stent components must be polished before they are placed in the body, and the rate of chemical polishing (in particular electrochemical polishing) of nickel titanium alloy can be very different from that of tantalum. This would indicate polishing separately the tantalum markers and the nickel titanium stent, but the chemical polishing process can disturb the accurate dimensional tolerance as achieved when these components are cut with a laser. One does not know with certainty how much material will have been removed from the intended welding interface by the chemical polishing procedures. Once the dimensions are disturbed, the certainty of optimal welding between the nickel titanium and the tantalum, at the welding interface, can be prejudiced, one desires to polish separately, then weld together with high precision and complete certainty.
Another publication, US 2005/0172471 A1, discloses how the change of nitinol between the austenite and martensite phases at the transition temperature can be used to encapsulate a marker element within a loop of shape memory alloy, with the loop contracting to engage the marker element in an interference fit due to the phase change. The principle is similar to thermo-mechanical interference fit techniques often used for joining, e.g., two metal components where one fits inside a loop or hole of the other, and relies on the shape change properties of the shape memory alloy at the transition point.